Caption: Most modern particle accelerators use electromag- nets to bend & focus a circulating beam of particles, which travel through an evacuated pipe. This is a view of a quadrupole magnet used to focus the beam, rather as a lens focuses a beam of light. It has two north poles & two south poles arranged so that poles of the same kind are diametrically opposite each other. The coils of wire that generate the magnetic field are clearly visible, wrapped in a cream-coloured covering. The beam pipe passes between the four faces of the magnet.